Uranium can be recovered from commercial grade wet process phosphoric acid by an oxidative extraction-reductive stripping process. In this process, an oxidized phophoric acid solution is contacted, generally in a multistage, countercurrent extractor, with an organic extractant solvent composition having an affinity for oxidized uranium values. This composition is usually a mixture of di(2-ethylhexyl)phosphoric acid (D2EHPA) and trioctylphosphine oxide (TOPO) in a solvent medium. After extraction, two phases are formed, namely an aqueous acid raffinate phase and an organic phase rich in oxidized uranium values. A small part of the acid raffinate can be contacted with iron and used as a reductive strip solution. Then, the organic phase is stripped of its uranium content, generally in a multistage countercurrent reductive stripper, and the stripped organic solvent is returned to the extraction system.
This stripping is enhanced, in the first cycle of the process, by the reduction of the uranium to the +4 state during the stripping with a reduced strip acid solution containing a high concentration of ferrous iron, +2 state. Elemental iron has been used to reduce the aqueous acid raffinate strip solution and provide the desired concentration of ferrous ion. This reductive stripping is taught by Hurst et al., in U.S. Pat. No. 3,711,591. The pregnant aqueous stripping solution exiting the reductive stripper is then again oxidized after which it is fed into the second cycle of the uranium recovery process. In the second cycle, the oxidized uranium in the aqueous solution is again extracted into an organic and the organic is cycled to an ammonium carbonate stripping and precipitating stage. An iron ammonium-phosphate precipitate is sometimes formed in the second cycle extractor. Wiewiorowski et al., in U.S. Pat. No. 4,105,741, eliminated this precipitate, which interferes with uranium recovery, by washing the second cycle solvent, after exiting the second cycle stripper, with an outside stream of a purified acid selected from sulfuric, hydrochloric, nitric or iron-free phosphoric acid.
We have found that one of the factors affecting operation, and particularly the efficiency of the reductive stripping operation, is the lowering of the phosphoric acid concentration in the reduced acid strip solution fed into the first cycle reductive stripper. This has a deteriorative effect on the strip coefficient, and the eventual recovery of uranium. A major cause of the lowered concentration is believed to be water contamination from rain water and from pump seals and line or vessel cleaning. One solution has been to purge the first cycle circuit of the low P.sub.2 O.sub.5 strip solution. However, this generally requires a 48 to 72 hour period, during which time a considerable quantity of uranium will be lost. Another solution is to further increase the iron addition to increase the ferrous ion concentration in the reductive stripping solution. However, this increases the precipitation of iron ammonium-phosphates in the reductive stripper and in the second cycle extractor, necessitating more frequent shutdowns to remove the precipitates. What is needed is an inexpensive, on stream method to increase the strip coefficient of low P.sub.2 O.sub.5 strip solution, not requiring costly downtime.